This invention relates generally to additive manufacturing and, in particular, to nozzle configurations, and methods of use, in conjunction with laser-based direct metal deposition.
As disclosed in commonly assigned U.S. Pat. No. 6,122,564, the entire contents of which are disclosed herein by reference, direct metal deposition (DMD(trademark)) is a laser-based fabrication process capable of producing near net-shape, fully dense molds, dies, and precision parts, as well as engineering changes or repairs to existing tooling or parts. According to the process, an industrial laser beam is focused onto a workpiece, creating a melt pool into which powdered metal is injected. The beam is moved under CNC control, based on a CAD geometry, tracing out the part, preferably on a layer-by-layer basis. Optical feedback is preferably used to maintain tight control over the process.
An integral part of the DMD(trademark) process is the deposition nozzle used to deliver the metal powders to the melt pool. The nozzle must provide consistent and accurate control of the metal powder, which has a direct impact on the metallurgical properties, surface finish, and efficiency of the process. Existing nozzles for metal powder deposition or laser cladding have very low efficiencies, or catchment of powder being deposited. This results in excess powder on the workpiece, more frequent additions of powder in the storage devices, and higher costs. The efficiencies of laser based powder metallurgy nozzles are typically 15% efficient, meaning of the total volume of powder delivered to the melt pool only 15% of that powder is deposited.
A laser spray nozzle assembly is described in U.S. Pat. No. 4,724,299. The assembly includes a nozzle body with first and second spaced apart end portions. A housing, spaced from the second end portion, forms an annular passage. A cladding powder supply system is operably associated with the passage for supplying cladding powder thereto so that the powder exits the opening coaxial with a laser beam.
In operation, this nozzle has been found to exhibit a very low deposition efficiency. Other drawbacks include insufficient cooling through the nozzle (primarily the inner tip), powder supply and feed tubes which tend to be too restrictive and exposed to reflected laser beams, frequent clogging as the powder exits the nozzle towards the workpiece, no means of automated clog detection, and poor surface quality. Thus, the need remains for a nozzle exhibiting higher deposition efficiencies while solving some or all of these stated problems.
This invention resides in a nozzle, particularly suited to direct metal deposition, In operation, the improved nozzle increases the quality of metallurgical properties, enhances deposition rate and process efficiency, and improves surface quality, reliability, and maintainability. The inventive nozzle also permits multiple configurations, thereby creating a range of clearances from the workpiece. This allows flexibility for repair processes that require clearances to access deep cavities in a work piece.
In terms of construction, a nozzle assembly according to the invention includes a body having a central axis and a distal end terminating in a distal tip through which a laser beam emerges. A gas-carried powder feed path terminates in one or more powder outlets arranged in a first concentric ring surrounding the laser beam, and a shaping-gas inlet and one or more shaping-gas outlets are arranged in a second concentric ring surrounding the laser beam. The internal geometry of the body is such that the laser beam, powder, and shaping gas all converge substantially within a localized region of a workpiece spaced apart from the distal tip at a working distance.
In the preferred embodiment, the assembly further includes a powder splitter that evenly distributes powder flow into a plurality of tubes within the body, and an inner tip surrounding the laser beam, the distal tip including conical end against which the powder is urged prior to exiting the body to assist in directing the powder to the localized point of the workpiece. The shaping gas preferably exits the body in a laminar flow condition.
In the preferred embodiment as well, at least a portion of the distal tip is adjustable to alter the working distance from between 10 and 30 mm, for example. The preferred nozzle also includes a coolant input and output and a cooling circuit within the body that promotes turbulent flow of the coolant for improved heat transfer, and one or more contact thermocouples in communication with a controller for terminating the operation of the nozzle and/or generating an alarm if a desired operating condition is not met. The body is preferably constructed out of a highly thermally conductive material such as copper, or an alloy thereof, and surfaces of the body exposed to powder flow are electroplated with hard chromium.